The present invention relates generally to optical devices. The present invention relates more particularly to an apparatus and method for filtering electromagnetic radiation, such as optical communication signals used in dense wavelength-division multiplexing optical communication systems and the live.
Optical communication systems which utilize wavelength-division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) technologies are well known. According to both wavelength-division multiplexing and dense wavelength-division multiplexing, a plurality of different wavelengths of light, typically infrared light, are transmitted via a single medium such as an optical fiber. Each wavelength corresponds to a separate channel and carries information generally independently with respect to the other channels. The plurality of wavelengths (and consequently the corresponding plurality of channels) are transmitted simultaneously without interference with one another, so as to substantially enhance the transmission bandwidth of the communication system. Thus, according to wavelength-division multiplexing and dense wavelength-division multiplexing technologies, a much greater amount of information may be transmitted than is possible utilizing a single wavelength optical communication system.
The individual channels of a wavelength-division multiplexed or dense wavelength-division multiplexed signal must be selected or separated from one another at a receiver in order to facilitate detection and demodulation thereof. This separation or demultiplexing process can be performed by an interleaver. A similar device facilitates multiplexing of the individual channels by a transmitter.
It is important that the interleaver separate the individual channels sufficiently so as to mitigate undesirable crosstalk therebetween. Crosstalk occurs when channels overlap, i.e., remain substantially unseparated, such that some portion of one or more non-selected channels remains in combination with a selected channel. As those skilled in the art will appreciate, such crosstalk interferes with the detection and/or demodulation process. Typically, the effects of crosstalk must be compensated for by undesirably increasing channel spacing and/or reducing the communication speed, so as to facilitate reliable detection/demodulation of the signal.
However, as channel usage inherently increases over time, the need for efficient utilization of available bandwidth becomes more important. Therefore, it is highly undesirable to reduce communication speed in order to compensate for the effects of crosstalk. Moreover, it is generally desirable to reduce channel spacing so as to facilitate the communication of a greater number of channels.
Filters are typically used within interleavers (and are also used in various other optical devices), so as to facilitate the separation of channels from one another in a wavelength-division multiplexing or dense wavelength division multiplexing system. Various characteristics of such filters contribute to the mitigation of crosstalk and thus to contribute reliable communications. For example, the ability of a filter to separate one optical channel from another or one set of channels from another set of channels is dependent substantially upon width and depth of the filter""s stopband. Generally, the wider and deeper the stopband, the more effectively the filter rejects unwanted adjacent channels and thus the more effectively the filter mitigates crosstalk.
Further, the flatness and width of the filter""s passband is important. The flatness of the filter""s passband determines how much the signal is undesirably altered during the filtering process. A substantially flat passband is desired, so as to assure that minimal undesirable alteration of the signal occurs. The width of the passband determines how far from the ideal or nominal channel center frequency a signal can be and still be effectively selected. A wide passband is desirable because the nominal center frequency of a carrier which is utilized to define a communication channel is not perfectly stable, and therefore tends to drift over time. Further, the nominal center frequency of a filter passband likewise tends to drift over time. Although it is possible to construct a system wherein the center frequency of the communication channel and the center frequency of the filter are comparatively stable, it is generally impractical and undesirably expensive to do so.
In order to construct a system wherein the center frequency of the communication channel and the center frequency of the filter are comparatively stable, it is necessary to provide precise control of the manufacturing processes involved. Since it is generally impractical and undesirably expensive to provide such precise control during manufacturing, the center frequency of communication channels and the center frequency of filters generally tend to mismatch with each other. Precise control of manufacturing processes is difficult because it involves the use of more stringent tolerances which inherently require more accurate manufacturing equipment and more time consuming procedures. The center frequency of the communication channel and the center frequency of the filter also tend to drift over time due to inevitable material and device degradation over time and also due to changes in the optical characteristics of optical components due to temperature changes. Therefore, it is important that the passband be wide enough so as to include a selected signal, even when both the carrier frequency of the selected signal and the center frequency of the passband are not precisely matched or aligned during manufacturing and have drifted substantially over time.
Birefringent filters for use in wavelength-division multiplexing and dense wavelength-division multiplexing communication systems are well known. Such birefringent filters are used to select or deselect optical signals according to the channel wavelengths thereof. However, contemporary birefringent filters tend to suffer from deficiencies caused by inherent carrier and passband instability due to manufacturing difficulties and due to drifting over time, as discussed above. That is, the passband of a contemporary birefringent filter is not as flat or as wide as is necessary for optimal performance. Further, the stopbands of such contemporary birefringent filters are not as deep or as wide as is necessary for optimal performance. Therefore, it is desirable to optimize such birefringent filters in a manner which enhances the width of the passband, makes the passband more flat, and which also widens and deepens the stopband. Further, it is desirable to provide a birefringent filter whereby the width of the stopband is roughly equal to the width of the passband, so as to facilitate the efficient separation of equally spaced channels in a wavelength-division multiplexing or dense wavelength-division multiplexing communication system.
Such birefringent filters typically comprise a plurality of birefringent elements placed end-to-end between two polarization selection devices, so as to define a contemporary Solc-type optical filter.
Referring now to FIG. 1, a typical layout of a Solc-type filter is shown. This layout is common to both contemporary Solc-type filters and the present invention. This filter comprises an input polarization selection device (e.g., polarizer) 11, an output polarization selection device 12, and a birefringent element assembly disposed generally intermediate the input polarization selection device 11 and the output polarization selection device 12. The polarization axis of the input polarization selection device 11 and the output polarization selection device 12 are typically parallel to one another.
According to contemporary practice, the birefringent element assembly 13 of such a Solc-type filter comprises three birefringent elements or crystals. A first birefringent crystal 15 has a length of L. A second birefringent crystal 16 has a length of 2L. A third birefringent crystal 17 has a length of 2L.
Further, according to contemporary practice the orientations of the fast axes of the birefringent crystals 15, 16 and 17 with respect to the polarization axis of the input polarizer 11 (and typically with respect to the polarization axis of the output polarization selection device 12, as well), are 45xc2x0 for the first birefringent crystal 15, xe2x88x9215xc2x0 for the second birefringent crystal 16, and 10xc2x0 for the third birefringent crystal 17.
Although such contemporary Solc-type filters are generally suitable for some applications in optical communications, such contemporary Solc-type filters suffer from inherent deficiencies which detract from their overall effectiveness. Such contemporary Solc-type filters are birefringent filters which suffer from an insufficiently flat and undesirably narrow passband, as well as an insufficiently deep and undesirably narrow stopband, as discussed above.
Referring now to FIGS. 2 and 3, transmission vs. wavelength curves for both the present invention and contemporary filters are shown.
With particular reference to FIG. 2, the stopband of the contemporary filter has peaks which are 20 dB down from the 0 dB level of the passband. Thus, the illustrated contemporary Solc-type filter provides only approximately 20 dB of cutoff in the stopband thereof. Further, the contemporary filter has a comparatively narrow xe2x88x9230 dB stopband.
With particular reference to FIG. 3 (which shows the two curves of FIG. 2 with increased resolution), it can be seen that the passband of the contemporary Solc-type filter contains an undesirable amount of ripple, and therefore is not as flat as desirable. Thus, such a contemporary Solc-type filter undesirably alters a signal which is transmitted therethrough.
The comparatively large amount of ripple in the passband of the contemporary Solc-type filter, in combination with the insufficiently deep stopband thereof, substantially degrades the performance of the filter such that the contemporary Solc-type filter frequently cannot meet the desired performance requirement therefor. Such poor performance all too frequently facilitates undesirable crosstalk between adjacent channels in wavelength-division multiplexing and dense wavelength-division multiplexing communication systems, particularly in those systems wherein the carrier wavelengths and/or the passband/stopband positions of the filter are insufficiently stable and not well controlled.
In view of the foregoing, it is desirable to provide a filter which has a comparatively flat transmission vs. wavelength characteristic curve at that portion of the curve defining the passband and which also has a comparatively deep stopband, so as to substantially mitigate crosstalk and so as to enhance filter performance in wavelength-division multiplexing, dense wavelength-division multiplexing, and similar communication systems.
The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, the present invention comprises a filter for filtering electromagnetic radiation, wherein the filter comprises two polarizing elements and a birefringent element assembly disposed generally intermediate the polarizing elements. The birefringent element assembly is preferably configured so as to optimize contributions of a fundamental and at least one odd harmonic of a transmission vs. wavelength curve in a manner which enhances transmission vs. wavelength curve flatness for a passband thereof. The birefringent element assembly is also preferably configured so as to optimize contributions of a fundamental and at least one odd harmonic of a transmission vs. wavelength curve in a manner which enhances the depth of the stopband thereof.
These, as well as other advantages of the present invention, will be more apparent from the following description and drawings. It is understood that changes in the specific structure shown and described may be made within the scope of the claims without departing from the spirit of the invention.